In various types of electronic equipment, such as mobile terminals, typified by mobile telephones, a wide diversity of types of batteries are used as the power source. In addition, the strengthening of regulations on emissions of carbon dioxide gas and so forth against a background of increasingly vigorous movements for environmental protection in recent years has seen the automobile industry engaging energetically in development of electric vehicles (EVs) and hybrid electric vehicles (HEVs) as well as vehicles using fossil fuels such as gasoline, diesel oil, and natural gas. Furthermore, the steep rise in the price of fossil fuels in recent times has acted as an engine driving forward the development of these EVs and HEVs.
Well-known secondary batteries used for such applications are nonaqueous electrolyte secondary batteries typified by the lithium ion battery, alkali storage batteries typified by nickel-hydrogen batteries, and the like. Among them, the range of applications of nonaqueous electrolyte secondary batteries typified by the lithium ion battery is expanding due to their superior characteristics, examples of which are high actuation voltage (3V or higher) and high theoretical energy density compared to aqueous electrolyte batteries, along with low self-discharge, and furthermore a wide operating temperature range and excellent liquid leakage resistance.
Such secondary batteries are composed of, for example, a rolled electrode group including positive electrode plates and negative electrode plates that are rolled, with separators interposed, into a flattened form; a prismatic battery case that is of a size such as to house the rolled electrode group and a particular quantity of electrolyte, and that is open at the top; and a sealing body which is inserted into the open top of the prismatic battery case, thereby sealing such opening, and on which there are formed a positive electrode terminal and a negative electrode terminal. To assemble such a battery, first of all the flattened-form rolled electrode group is placed inside the prismatic battery case, then the collectors of the positive electrode plates and the negative electrode plates are welded to the positive electrode terminal and the negative electrode terminal respectively. Next, the sealing body is welded to the opening of the battery case, and electrolyte is poured in through a pour hole provided in the sealing body. Finally, the pour hole is sealed.
With such batteries however, short circuits may occur in the interior due to some cause or other. When such internal short circuits occur, they can not only result in product failure, but also induce bursting or ignition accidents. Nonaqueous electrolyte secondary batteries, typified by the lithium ion battery, are particularly prone to burst or ignite because they have high capacity and output characteristics, so that if internal short-circuits occur, chemical reactions may take place between the electrode body and nonaqueous electrolyte contained in the battery, resulting in abnormal pressure rise in the battery interior. Accordingly, for such batteries, technology has been put forward that prevents short circuits by affixing insulating tape at the locations where internal short-circuits are prone to occur (see for example JP-A-2000-188115 (paragraphs [0006] to [0007], FIGS. 3 and 9), JP-A-2006-128106 (paragraph [0008], FIGS. 2 and 3), JP-A-9-134719 (paragraph [0009], FIGS. 2 and 3), and JP-A-2001-76758 (paragraphs [0010] and [0011], FIG. 2)).
The battery short-circuit prevention technology disclosed in JP-A-2000-188115 and JP-A-2006-128106 will now be described, referring to FIGS. 6 and 7. FIG. 6A is a plain view of the electrode material end portion disclosed in JP-A-2000-188115, FIG. 6B is a cross-sectional view along line VIB-VIB in FIG. 6A, and FIG. 6C is a longitudinal cross-sectional view of the electrode material end portion given as an example of the related art in JP-A-2000-188115. FIG. 7A is a plain view of the electrode plate disclosed in JP-A-2006-128106, and FIG. 7B is a cross-sectional view along line VIIB-VIIB in FIG. 7A.
An electrode material end portion 50 disclosed in JP-A-2000-188115 is an improvement of an electrode material end portion 50A of the related art (see FIG. 6C). As FIGS. 6A and 6B show, in this electrode material end portion 50 there is formed an area 51a where an electrode active material layer 52 formed on an electrode collector 51 is removed, and by affixing resin tape 53 to this area 51a, a thickness H1 of the portion where the resin tape 53 is provided is rendered almost equal to a thickness H2 of the portion where the electrode active material layer 52 is formed.
With such configuration, the level difference between the portion where the resin tape 53 is formed and the portion where the electrode active material layer 52 is formed disappears, as is plain from a comparison with the electrode material end portion 50A shown in FIG. 6C. This means that with the invention disclosed in JP-A-2000-188115, the resin tape 53 yields the effect of preventing electrical short-circuits due to burr produced when the electrode material is cut, or to the level-difference portions, which arise between the end portions of the positive and negative electrodes on the one hand, and the electrode terminals on the other, when the latter are welded to the former, piercing through the thin gel-like electrolyte layer and touching the other electrode.
Also, in an electrode plate 54 disclosed in JP-A-2006-128106, protrusions a and a′ formed at the application start edge and application finish edge when an active material 56 is applied to the surface of an electrode material 55 are covered with insulating members 57, as FIGS. 7A and 7B show. The base members of these insulating members 57 are formed from a porous electrolyte-permeable material. Also, a collection tab 58 is provided in the region of the electrode plate 54 where the active material 56 is not applied.
Thus, thanks to the protrusions a and a′, which rise at the application start edge and application finish edge of the active material 56, being covered by the insulating members 57, damage to the separators due to contacting of the positive and negative electrodes with each other and with the protrusions a and a′ is prevented. Hence, while electrically insulating the two electrode plates, the insulating members 57 also exert the advantageous effect of preventing occurrence of electrical short-circuits between the two electrode plates, and decrease of the battery capacity, because they are formed from a porous material that facilitates movement of the electrolyte.
Further, also well-known are inventions whereby the application edge portions of the electrode mixture are covered with lithium ion-opaque tape, which suppresses reactions at the portions of the negative electrode that do not oppose the positive electrode, so that the storage characteristics of the nonaqueous electrolyte secondary battery are enhanced (see for example JP-A-9-134719), or whereby one peripheral edge of the positive electrode surface is covered with insulating tape so as to provide a nonaqueous electrolyte secondary battery in which contacting of the burr occurring on the cut surfaces of one electrode plate with the other electrode plate is prevented (see for example JP-A-2001-76758).
Employing an electrode material end portion configuration such as disclosed in the aforementioned JP-A-2000-188115, JP-A-2006-128106, JP-A-9-134719 and JP-A-2001-76758 will essentially yield the advantageous effect of preventing the short-circuits that are liable to occur between the electrodes of the nonaqueous electrolyte secondary battery fabricated.
However, the positive electrode plate and the negative electrode plate are both fabricated using an elongated strip-form electrode sheet, which undergoes a process of cutting to the appropriate electrode length after a layer of positive electrode active material or negative electrode active material, as appropriate, has been applied to the sheet by a predetermined method. There are two methods for application of the active material layer to the elongated electrode sheet. One is to apply just the amount required for formation of one electrode, then leave a substrate-exposed portion where the active material layer is not applied, and to repeat such alternating applications and non-applications of the layer, applying it “intermittently”, so to speak (this is termed the “intermittent application method” below). The other method is to apply the layer continuously, positioning the substrate-exposed portion at one of the edges that are orthogonal to the longitudinal direction (this is termed the “continuous application method” below). The substrate-exposed portion is provided in order to fix the collection tab.
When such continuous application method is employed, the active material layer and the collector, which supports the active material layer, are cut simultaneously during cutting of the elongated electrode sheet. Because of this, burr protrusions occur on the cut surfaces of the collector, and also, the cross-section of the active material layer and the regions close to such cross-section are put into an unstable state due to the impacts during cutting, so that the active material layer is prone to slide off. Whereas with the intermittent application method the problem of the active material layer sliding off due to the cutting does not arise, because the cutting is performed at the substrate-exposed portion. Nevertheless, with the intermittent application method protrusions do occur at the application starting portions and application ending portions in application of the active material layer, and it is therefore necessary to affix resin tape, in the manner set forth in JP-A-2006-128106.
In the subsequent assembly process, positive electrode plates and negative electrode plates with active material layers formed by employing the foregoing continuous application method are stacked or rolled together with separators interposed. During such process, there is a risk that the active material layer at the cut surfaces of the positive and negative electrode plates slides off due to impacts and the like, since such active material layer has become prone to slide off. If such active material layer does slide off, the slid active material migrates inside the battery interior after assembly of the battery, causing internal short-circuits. To prevent such sliding-off of the active material layer, a method that suggests itself is to affix adhesive tape to the cut surfaces of the electrode plates, as in the related art disclosed in JP-A-2000-188115 (see FIG. 6C). However when adhesive tape is affixed to the cut surfaces of the electrode plates, the adhesive provided on the tape may absorb electrolyte and cause the electrolyte inside the separators to deplete. Should the electrolyte inside the separators deplete in this way, the positive electrode material sticks out, causing an internal short circuit.
Considered from this aspect, affixing tape to the electrode material end portions with adhesive in an example of related art entails the risk that the adhesive applied to the tape absorbs electrolyte and cause the electrolyte inside the battery to deplete. However, in the inventions disclosed in the aforementioned JP-A-2000-188115, JP-A-2006-128106, JP-A-9-134719 and JP-A-2001-76758, no consideration whatever is given to the problem of depletion of the electrolyte when tape is affixed to the electrode material end portions with adhesive.
For example, with the electrode material end portion 50A of the related art set forth in JP-A-2000-188115 (see FIG. 6C), the adhesive of the resin tape is stuck over the whole surface of the active material, so that there is a risk that the adhesive absorbs electrolyte and deplete the electrolyte inside the separators. On the other hand, the configuration of the electrode material end portion 50 (see FIGS. 6A and 6B), corresponding to the improved invention disclosed in JP-A-2000-188115, represents the case where, after an elongated electrode plate fabricated using the continuous application method has been cut at the portion where the active material layer is present, a substrate-exposed portion is provided by removing the active material layer formed on the substrate, and in this case there is risk that the active material layer slides off because its end portions are not covered with the resin tape. Also, in the case where substrate-exposed portions are provided via the intermittent application method and cutting is then performed at such portions, the active material layer is not cut, and so the problem of the active material layer sliding off, which arises if the active material layer is cut, is absent. Note that the structure of the electrode material end portion of JP-A-2006-128106 is obtained by providing substrate-exposed portions via the intermittent application method, then performing cutting at such portions, so that the active material layer is not cut, and hence with such structure the problem of the active material layer sliding off, which arises if the active material layer is cut, is absent. JP-A-9-134719 and JP-A-2001-76758 also make no mention whatever concerning the sliding-off and migration of the active material that occurs if the active material layer is cut, or concerning depletion of the electrolyte.